1. Technical Field
The present invention relates to a DC-DC converter, and more particularly, to a non-isolated DC-DC converter that can operate in either PWM control mode or VFM control mode depending on load conditions with increased power efficiency during VFM-controlled operation.
2. Discussion of the Background
With currently increasing concern for environmental and ecological issues, there is a growing need for energy-saving electronic devices, particularly those operating with battery-based power supply. Two common approaches to conserving energy in electronic equipment are to reduce power consumed by functional units of the electronic device, and to improve efficiency and reduce energy loss of power circuitry supplying the electronic device.
A non-isolated DC-DC converter is an example of a high-efficiency power supply used in small portable electronic devices, in which an output transistor switches on and off current flow to temporarily store an input energy in an inductor and release the stored energy at a constant voltage higher or lower than that of the input energy.
One major technique used to control operation of a DC-DC converter is pulse width modulation (PWM) control, which adjusts an ON time or duty cycle of a switching transistor with a clock signal having a constant frequency and a variable pulse width or pulse duration. Another control technique commonly used is variable frequency modulation (VFM) control, also referred to as pulse frequency modulation (PFM) control, which adjusts a switching frequency of a switching transistor with a clock signal having a variable frequency and a constant pulse duration. Some VFM control schemes vary a clock frequency seamlessly using a variable oscillator, and others create a variable-frequency signal by skipping pulses in an original pulse train oscillating at a constant frequency.
Typically, power consumed by a DC-DC converter increases in proportion to the switching frequency with which a switching transistor is operated. That fact indicates that with low power supplied to load circuitry, PWM control is less efficient in terms of power consumption than VFM control, since the former constantly switches the output transistor at a fixed frequency whereas the latter can adjust the switching frequency to decrease corresponding to the low-load condition. On the other hand, VFM control is inferior to its counterpart in terms of operational stability, for varying the switching frequency can cause noise and ripple in the output of the DC-DC converter.
To combine the advantages of PWM and VFM control schemes, some conventional voltage regulators incorporate power supply circuitry that can switch control mode between PWM mode and VFM mode. These dual-control mode voltage regulators, operating in VFM control mode under low load and in PWM control mode under high load, provide high power efficiency over a wider range of operating conditions than is possible with a single-control mode power supply.
What is required for good performance of a PWM/VFM-controlled DC-DC converter is to properly determine the timing at which the control mode switches between PWM and VFM modes, and in particular, to synchronize transition of the control mode with transition of the output current so that transition from VFM mode to PWM mode occurs exactly when the output current reaches a threshold value with which the DC-DC converter enters continuous mode operation. Moreover, to obtain even higher efficiency of dual-control mode voltage regulation, it is desirable to set the switching frequency as low as possible during VFM-controlled operation.
Several techniques have been proposed to determine the timing at which a dual-control mode switching transistor switches the control mode from VFM mode to PWM mode according to load conditions.
For example, one conventional method proposes a DC-DC converter that determines the transition timing by monitoring the load condition according to an error amplifier indicating the level of the output voltage. Another conventional method proposes a VFM/PWM-controlled power supply that can switch the control mode by detecting the load condition from an error amplifier output and perform VFM mode control without using a voltage-controlled oscillator.
The conventional methods described above have a fixed ON-time of the switching transistor during VFM-controlled operation, upon which the switching frequency is dependent. This results in difficulty in adjusting the switching frequency during VFM-controlled operation where the VFM/PWM transition occurs with the output current reaching the threshold current.
Thus, what is needed is a dual-control mode DC-DC converter that can smoothly switch control mode from VFM to PWM without causing variations in the output voltage, while improving power efficiency during VFM-controlled operation by reducing the switching frequency under low-load conditions.